The present invention relates to apparatus for processing web, bands or strips of photographic films, photographic paper or like photosensitive material. More particularly, the invention relates to improvements in apparatus for wetting and thereupon drying running webs of photosensitive material, especially photographic paper. Such apparatus can be utilized in photographic processing laboratories for customer films wherein webs of exposed photographic paper which carry latent images of customer film frames are transported through a developing machine to contact the webs with developing, fixing and rinsing fluids prior to passage through a drying unit wherein the liquid which is entrained from the last bath is caused to evaporate before the webs are convoluted onto reels or advanced directly to a severing and classifying station.
It is already known to transport webs of wet photographic paper through a drying unit wherein each web advances along a substantially U-shaped path. The drying unit has room for simultaneous transport of several webs along discrete paths, and the width of all webs may but need not be the same. The temperature of air in the drying unit is regulated to remain within a selected range, and the drying unit is further equipped or associated with means for withdrawing spent air from its interior. A blower is employed to circulate the air in the drying unit. The air stream or streams which issue from the outlet of the blower are heated prior to admission laterally into a distributor which is installed at the bight of the U-shaped path. The distributor has orifices which enable heated air to form jets impinging upon the adjacent portions of the running webs. The means for regulating the temperature of air in the drying unit comprises a control device which can turn a heater on or off, depending upon whether the temperature of air drops below the lower limit or rises above the upper limit of the range of acceptable temperatures. The means for withdrawing spent air comprises an exhaust fan below the top wall of the drying unit, and the top wall has an opening for admission of relatively dry atmospheric air. The opening contains an adjustable blind or shutter whose setting determines the quantity of inflowing atmospheric air.
A drawback of the just described conventional apparatus is that it operates satisfactorily only when the quantity of web material in the drying unit matches or approximates a predetermined value. On the other hand, the quantity of web material in the drying unit often fluctuates within an extremely wide range. For example, and assuming that the apparatus defines three discrete paths for webs of maximum width, for webs of medium width and for narrow webs, the quantity of web material is only a minute fraction of the maximum quantity when the apparatus processes a single web, especially a single web of minimum width. Furthermore, the speed at which the web or webs are transported through the drying unit is also likely to fluctuate within a wide range. Moreover, certain webs are likely to attract and entrain relatively large quantities of a liquid processing medium whereas the percentage of liquid which is entrained by a web of a different type is very low.
Since the just described apparatus maintains the temperature of air in the drying unit at a constant value, the drying or moisture-removing action of such air upon a single web is different than upon a set of two or more webs which travel, at the same time, from the inlet to the outlet of the drying unit. By maintaining the temperature of air at a constant value, the just described conventional apparatus cannot undergo adjustments which are necessary when one or more webs of a first type are followed by one or more webs of a different second type or vice versa. An additional drawback of conventional apparatus is that the distribution of heated air upon all surfaces of webs in the drying unit is not uniform; this, too, affects the quality of the drying action. For example, the streams of heated air impinge upon the webs in different directions and often maintain layers of moisture-laden air in long-lasting contact with the webs so that the webs cannot be contacted by air having a lower moisture content. The adjustment of the aforementioned blind is time-consuming and the mounting of the exhaust fan in the interior of the drying unit also presents problems.